This invention relates to processes for the conversion of aromatic hydrocarbons, and is more specifically an improved process for disproportionation and/or transalkylation of aromatic hydrocarbons to obtain xylenes.
Xylene isomers are produced in large volumes from petroleum as feedstocks for a variety of important industrial chemicals. The most important: of the xylene isomers is paraxylene, the principal feedstock for polyester which continues to enjoy a high growth rate from large base demand. Orthoxylene is used to produce phthalic anhydride, which has high-volume but mature markets. Metaxylene is used in lesser but growing volumes for such products as plasticizers, azo dyes and wood preservers. Ethylbenzene generally is present in xylene mixtures and is occasionally recovered for styrene production, but usually is considered a less-desirable component of C8 aromatics.
Among the aromatic hydrocarbons, the overall importance of the xylenes rivals that of benzene as a feedstock for industrial chemicals. Neither the xylenes nor benzene are produced from petroleum by the reforming of naphtha in sufficient volume to meet demand, and conversion of other hydrocarbons is necessary to increase the yield of xylenes and benzene. Most commonly, toluene is dealkylated to produce benzene or disproportionated to yield benzene and C8 aromatics from which the individual xylene isomers are recovered. More recently, processes have been introduced to disproportionate toluene selectively to obtain higher-than-equilibrium yields of paraxylene.
A current objective of many aromatics complexes is to increase the yield of xylenes and to deemphasize benzene production. Demand is growing faster for xylene derivatives than for benzene derivatives. Refinery modifications are being effected to reduce the benzene content of gasoline in industrialized countries, which will increase the supply of benzene available to meet demand. Benzene produced from disproportionation processes often is not sufficiently pure to be competitive in the market. A higher yield of xylenes at the expense of benzene thus is a favorable objective, and processes to transalkylate C9 aromatics along with toluene have been commercialized to obtain high xylene yields.
U.S. Pat. No. 4,097,543 (Haag et al.) teaches toluene disproportionation for the selective production of paraxylene using a zeolite which has undergone controlled precoking. The zeolite may be ion-exchanged with a variety of elements from Group IB to VIII, and composited with a variety of clays and other porous matrix materials.
U.S. Pat. No. 4,203,869 discloses a zeolite having an aluminum-free outer shell of SiO2 which has the same crystal structure as the zeolite. The zeolite is made by a two-stage method of initiating crystallization of the zeolite, then altering the crystallization medium to eliminate the aluminum moiety.
U.S. Pat. No. 4,861,739 discloses a multiphase, multicompositional composite, at least one phase of which is a “QAPSO” non-zeolitic molecular sieve (NZMS) comprising phosphorus, aluminum and another element Q. Much of the benefits of NZMSs as catalysts are achieved in the outer portion of a particle, and secondary reactions in the core of a particle are avoided with this invention.
U.S. Pat. No. 4,482,774 (W. T. Koetsier) presents hydrocarbon conversion processes, including toluene disproportionation, which are performed using a catalyst having a core of silica with an overlying modified-silica zeolite, having substantially the same crystalline structures. The core is described as having only a few acid sites and little catalytic activity, and preferably having a ratio of silicon to modifying elements above 500. The core may have an MFI structure. The modified silica shell of this catalyst may have many compositions including a specified borosilicate and gallosilicate.
Workers in the field of aromatics disproportionation continue to seek processes and catalysts having exceptionally high selectivity for paraxylene from toluene combined with favorable activity and stability.